Light olefins, such as ethylene, propylene and butylenes, can be produced using various processes such as steam cracking, fluid catalytic cracking, conversion of methanol to olefins, paraffin dehydrogenation, alcohol dehydration, methane coupling and Fischer Tropsch reactions. However, these processes often produce varying levels of acetylenic and/or diene by-products, such as acetylene, methyl acetylene (MA), propadiene (PD), butyne and butadiene. These by-products must be removed from the light olefin streams because they can act as poisons to the downstream processing catalysts, such as polymerization catalysts.
The preferred method of removing these by-products is by selective catalytic hydrogenation in which, for example, acetylene is converted to ethylene, methyl acetyene and propadiene are converted to propylene, and butyne and butadiene are converted to butylenes. Possible competing reactions include the oligomerization of two or more alkyne or diolefin molecules to produce heavier unsaturated compounds (generally referred to as “green oil”) and the generation of saturates (for example, ethane, propane and butanes) as a result of over-hydrogenation. Both of these reactions are undesirable in that they reduce the selectivity to the required light olefins. However, the green oil is particularly problematic in that it decreases the life of the hydrogenation catalyst.
Currently, the catalysts proposed for this selective hydrogenation comprise at least one Group 8–10 metal, such as palladium or rhodium, often in combination with other metals, such as indium, wherein the metals are dispersed on a support, such as alumina. However, the activity and selectivity of such catalysts are normally dependent not only on the type of metal(s) employed but also on the ability to disperse or alloy small particles of the desired metal(s) homogeneously on the support. Typically supported metal catalysts are prepared by impregnation of the support with solutions containing salts of the desired metals, followed by drying and calcination. However, impregnation, and particularly slurry impregnation, can lead to non-homogeneous metal dispersion or alloying, particularly at high metal loadings.
There is therefore a need for an improved method of dispersing/alloying metals on a catalyst support such that the resultant catalyst exhibits improved performance in the selective hydrogenation of alkynes and/or diolefins.
U.S. patent application Publication No. 2002/0068843 discloses a catalyst for selectively hydrogenating acetylenic and diolefinic compounds with low green oil formation, the catalyst comprising the following active components loaded on a porous inorganic support: (1) at least one of platinum, palladium, nickel, ruthenium, cobalt, and rhodium; (2) at least one of silver, copper, zinc, potassium, sodium, magnesium, calcium, beryllium, tin, lead, strontium, barium, radium, iron, manganese, zirconium, molybdenum, and germanium; (3) at least one rare earth metal selected from scandium, yttrium and Lanthanides in Group IIIB of Periodic Table of Elements; and (4) bismuth. Preferably, component (1) is platinum or palladium component (2) is silver, potassium or sodium and component (3) is lanthanum or neodymium.
U.S. Pat. No. 6,255,548 discloses a method for selectively hydrogenating a feed comprising an acetylenic compound and/or a diolefin in the presence of a catalyst comprising at least one support, at least one Group VIII metal selected from nickel, palladium, platinum, rhodium, ruthenium and iridium and at least one additional element M selected from germanium, tin, lead, rhenium, gallium, indium, thallium, gold, and silver, wherein the catalyst is formed by impregnating the support with an aqueous solution containing at least one water-soluble organometallic compound of said element M comprising at least one carbon-M bond. The preferred Group VIII metals are nickel, palladium and platinum and the preferred additional elements M are germanium, tin, gold, and silver.
U.S. Pat. No. 5,877,363 discloses a process for the removal of acetylenes and 1,2-butadiene from a C4 aliphatic hydrocarbon stream by contacting the hydrocarbon stream with hydrogen in a distillation column reactor containing a bed of hydrogenation catalyst comprising a Group VIII metal selected from platinum, palladium, rhodium or mixtures thereof; optionally in combination with a Group IB or Group VIB metal, and fractionally distilling the reaction mixture to remove a heavier fraction and removing a fraction overhead comprising substantially all of the C4 compounds having reduced acetylenes and 1,2-butadiene content. The preferred hydrogenation catalyst is palladium.
U.S. Pat. Nos. 5,356,851 and 5,364,998 disclose a catalyst and a process for the selective hydrogenation of unsaturated compounds, wherein the catalyst contains 0.1 to 10% of at least one Group VIII metal selected from nickel, palladium, platinum, rhodium and ruthenium and 0.01 to 10% of at least one Group IIIA metal selected from gallium and indium. The molar ratio of Group IIIA metal to Group VIII metal is between 0.2 and 5, preferably between 0.3 and 2. The metals are deposited on a catalyst support by (a) impregnating the support with a solution of a Group IIIA metal compound precursor, then (b) impregnating the product of (a) with a solution of a Group VIII metal compound and then (c) calcining the product of (b) at 110 to 600° C. The preferred Group VIII metals are nickel, palladium and platinum.
Published International Application No. WO 98/47618 discloses a process for producing a Fischer-Tropsch catalyst wherein a multi-functional carboxylic acid having 3 to 6 carbon atoms is used to impregnate and disperse a rhenium compound and a compound of a catalytic metal, such as copper or iron, onto a refractory oxide support, such as titania. Published International Application Nos. WO 98/47617 and WO 98/47620 disclose similar processes but in which the dispersion aids are a polyol and a carbohydrate or sugar respectively.
United Kingdom Patent Application No. 0227086.6 (Attorney Docket No. 01CL123), filed Nov. 20, 2002, discloses a method for preparing a metal supported catalyst in which metal dispersion is improved by forming an organic complex of at least one catalytically active metal during manufacture of the catalyst and then partially or fully decomposing the complex before reduction of the metal with hydrogen. The catalyst is said to be useful in catalyzing Fischer-Tropsch reactions and in removing organosulfur compounds from hydrocarbon streams. There is no disclosure of use of the catalyst in the selective hydrogenation of alkynes and/or diolefins.
Co-pending U.S. patent application Ser. No. 10/720,617 filed Nov. 24, 2003, describes a catalyst and process for selectively hydrogenating alkynes and/or diolefins, wherein the catalyst comprises a support, a rhodium component present in an amount such that the catalyst composition comprises less than 3.0% of rhodium by weight of the total catalyst composition; and an indium component present in an amount such that the catalyst composition comprises at least 0.4% and less than 5.0% of indium by weight of the total catalyst composition. The rhodium and indium components can be added to the support by impregnation or co-precipitation.
Co-pending U.S. patent application Ser. No. 10/720,558 filed Nov. 24, 2003, describes a catalyst and process for selectively hydrogenating alkynes and/or diolefins, wherein the catalyst comprises at least two different metal components selected from Groups 8 to 10 of the Periodic Table of Elements and at least one metal component selected from Group 13 of the Periodic Table of Elements. The metal components can be added to the support by impregnation or co-precipitation.